[flang-rt] Add the ability to have user supplied callback functions to further customize the runtime environment. (#155646)

Add the ability to have pre and post call back functions to
ExecutionEnvironment::Configure() to allow further customization of the
flang runtime environment (called from _FortranAStartProgam) in
situations where either the desired features/functionality are
proprietary or are too specific to be accepted by the flang community.

Example:
Custom constructor object linked with flang objects:
```
#include "flang-rt/runtime/environment.h"
#include "flang/Runtime/entry-names.h"
#include "flang/Runtime/extensions.h"

namespace Fortran::runtime {

// Do something specific to the flang runtime environment prior to the
// core logic of ExecutionEnvironment::Configure().
static void
CustomPreConfigureEnv(int argc, const char *argv[], const char *envp[],
                      const EnvironmentDefaultList *envDefaultList) {
  puts(__func__);
}

// Do something specific to the flang runtime environment after running the
// core logic of ExecutionEnvironment::Configure().
static void
CustomPostConfigureEnv(int argc, const char *argv[], const char *envp[],
                       const EnvironmentDefaultList *envDefaultList) {
  puts(__func__);
}

void __attribute__((constructor)) CustomInitCstor(void) {
  // Possibilities:
  // RTNAME(RegisterConfigureEnv)(&CustomPreConfigureEnv,
  // &CustomPostConfigureEnv); RTNAME(RegisterConfigureEnv)(nullptr,
  // &CustomPostConfigureEnv);
  RTNAME(RegisterConfigureEnv)(&CustomPreConfigureEnv, nullptr);
}
} // namespace Fortran::runtime
```

---------

Co-authored-by: David Parks <dparks@nvidia.com>